[noname]

Do you think it would be possible for a single element to be stable above bismuth?  I believe so because Technetium and Promethium are the only radioactive elements below Bismuth, so why not one or two elements happen to be stable in the other direction?

One thing we know is it would have to have an even number.

[Alberto_Kravina]

The only thing that I can say is that some scientists believe in an "island of stability" around 126..

[jdurg]

The only thing that I can say is that some scientists believe in an "island of stability" around 126..

But that island of stability is only in relation to their half-life.  So instead of a half-life on the order of a few milliseconds, the half life would be on the order of a few seconds.  

I am 99.9999999% certain that there are no stable elements with an Atomic Number greater than 83.  Pm and Tc are radioactive because they cannot obtain a stable configuration in their nucleus.  Either the nucleus is too "heavy" which results in so much strain that the nucleus falls apart, or there aren't enough neutrons to balance out the number of protons in the nucleus.  (Remember, like charges repel and the more protons you try and force into a nucleus the greater the repulsion is felt).  Neutrons act as a 'buffer' of sorts and are able to stabilize a nucleus.  If you have too many neutrons in that nucleus, however, it becomes more unstable due to the amount of "stuff" being forced into the nucleus.  Tc and Pm just aren't able to find the right configuration of neutrons to protons without the nucleus size becoming too large to be stable.

When you get above element 83, it becomes impossible to get the proper "configuration".  No matter what you do, all nuclei will either be too proton rich, or too massive in size to remain stable.  Hence they are all radioactive.

[Mitch]

Uranium-238 with a half-life of 4 Giga Years seems stable enough.

[constant thinker]

Uranium-238 with a half-life of 4 Giga Years seems stable enough.

  4 Billion years!!!! Jeese that is pretty stable. That must throw off barely detectable radiation if it can even be detected at all.

[Grejak]

4 Billion years!!!! Jeese that is pretty stable. That must throw off barely detectable radiation if it can even be detected at all.

Yeah, a gram of it will only give off about 12 counts per second, so you do not have to worry about it too much.

In regards to the original question, various theories have predicted an "Island of Stability" around N=184 and Z=114,120 or 126 (If I am remembering correctly).  The half-life predictions range from a few minutes to millions or billions of years.  If you extrapolate from what we do know about superheavy elements, it is very unlikely that the half-lives will be more than a few seconds.  That said, there is no conceivable way to reach that heavy of nuclei with current technology.  The closest isotopes to the island of stability that have been claimed so far are ²⁹²116 with 176 neutrons or ²⁹²114 with 178 neutrons.  Both still quite short of the doubly magic number.  

[jdurg]

Uranium-238 with a half-life of 4 Giga Years seems stable enough.

True indeed, but I wouldn't classify it as stable.  You can take a sample of Cl-35 and you'll see that it won't give off any radiation.  That is what I would determine as stable.  What can I say, I'm anal-retentive about things.  

[limpet chicken]

I would have thought retaining radioactive materials (or in your case, any isotope of chlorine) up there, would be quite bad for your health

[Mitch]

Even a proton will decay on a long enough time scale.

[wereworm73]

The doubly magic ²⁹⁸114 nuclei might last about 1000 years, not stable but pretty darn good for a superheavy element.  Problem is, aside from our limited technology, that we don't even have the pieces to put this sucker together.  The closest we could realistically get is to fuse curium-250 with calcium-48, but even if all the neutrons stay together, that still gives you ²⁹⁸116, which would probably alpha decay to ²⁹⁴114.

[Mitch]

You might be able to get the 3n exit channel. No one tries to search for 3n channels with projectiles with a larger Z than Carbon on Actinide targets, but it could be possible.